1. Field of Invention
This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to an electrochemical cell whose anode is composed of lithium or some alloy thereof. The cathode is composed of silver vanadium oxide (SVO) and carbon monofluoride (CFx) and the cell is particularly useful in implantable medical devices that require a pulsatile power source, such as an implantable cardioverter defibrillator (ICD). The invention specifically relates to the ratio of the capacity of the anode material in the cell to the capacity of the respective cathode materials.
2. Prior Art
Implantable cardioverter defibrillators are typically powered by cells containing a lithium anode and a silver vanadium oxide cathode. The Li/SVO cell chemistry provides high energy density, excellent reliability, and the high power pulse capability required by the defibrillator application. In certain cases, however, these cells exhibit middle-of-life voltage delay accompanied by a permanent increase in impedance (Rdc growth). This results in an increased time to charge the ICD's capacitors. Time between detection of a heart arrhythmia and therapy is critical to the effectiveness of an ICD. Thus, there is potential for impedance growth in the Li/SVO cell that limits the effectiveness of the implantable device.
In addition, it is desirable to use discharge voltage to indicate when the cell is nearing the end of its life. This is because it is important to have sufficient time and discharge capacity between the point at which the user is warned that the cell must be replaced and the time at which the cell is no longer functional. Failure to do so could result in device failure if the patient is unable to replace the device power source in time. The current state-of-the art Li/SVO power source discharges in two distinct voltage plateaus. Under some circumstances, it is difficult to select an appropriate replacement voltage because the cell may rapidly lose pulse capability after the second discharge plateau. To avoid that, it is desirable to select a voltage on or above the second discharge plateau. However, this results in a substantial loss of useful capacity, which, in turn, adversely impacts cell longevity.
These problems can be mitigated by using an excess of cathode material in the cell so that the anode is consumed before the cell voltage drops to the second discharge plateau. However, the extra cathode material consumes space resulting in decreased energy density. Thus, either the cell longevity is reduced or a larger cell must be used. Both compromises are undesirable in an implantable medical application. It is desirable, therefore, to have a cell with low impedance growth, a clear replacement indicator and high energy density.